Inhibitive action of the octyl esters of 4- and 5-carboxybenzotriazole for copper corrosion in sulphate solutions

Abstract The inhibitive action of a mixture of the octyl esters of 4- and 5-carboxybenzotriazole (4-CBTAH-OE and 5-CBTAH-OE) for copper in aerated 0.5 M sulphate solution over a range of pH values (0–8) was investigated. Coupon tests showed that at pH∼0 the inhibition efficiency of the mixture was approximately 98% at the 1×10−4 M level. This is considerably higher than that for benzotriazole (BTAH) (∼50%) under similar conditions. At pH∼8 the inhibition efficiency is still appreciable (∼75%) indicating that the mixed esters (in contrast to BTAH) have practical application for the inhibition of copper corrosion over a relatively wide pH range. Over the pH range no hydrolysis of the esters was observed. Surface enhanced Raman scattering (SERS) indicated that the esters, like BTAH, inhibit copper corrosion at low pH by chemisorption of the protonated species (CBTAH+2-OE) on the metal through an azole nitrogen. The increased inhibition at low pH is attributed to van der Waals’ forces of attraction between adjacent octyl chains oriented away from the copper surface. SERS also suggest that at higher pH chemisorption is replaced by the deposition of a sterically hindered polymeric complex that, in contrast to BTAH, is not as protective as chemisorbed CBTAH+2-OE. Potentiodynamic polarisation also supports the changeover to a different protective mechanism as the pH is raised.